This invention relates to the halogenation of cyclobutenoarenes. In one embodiment, the invention relates to recovery of 4-halobenzocyclobutene from a reaction product mixture containing by-product aliphatic halides such as 1-halo-2-(2-haloethyl)benzene.
The four-membered ring of benzocyclobutenes is known to open at elevated temperature to form a very reactive diene which rapidly dimerizes and polymerizes. Molecules containing two or more benzocyclobutene groups are therefore useful as heat-curable thermosetting resins. Also, elastomers or thermoplastics containing benzocyclobutene substituents crosslink on heating. One of the easiest and most useful methods of functionalizing the benzocyclobutene molecule for subsequent production of resins or functionalized polymers is halogenation of the aromatic ring. Bromination with aqueous bromine, for example, can yield 4-bromobenzocyclobutene in over 75% yield. However, a significant proportion of the starting benzocyclobutene (10-15%) reacts with bromine by addition and opening of the four-membered ring to form 1-bromo-2-(2-bromoethyl)benzene.
The simplest method of separating the desired 4-bromo product from the ring-opened dibrominated 1-bromo by-product would appear to be distillation. However, when the product mixture is distilled, even at pressures as low as 400 Pa (3 mm Hg), the distillation is eventually disrupted by HBr formation. HBr formation is also undesirable because of its corrosive effects on down-stream vessels and equipment. As the HBr is formed, the distillate becomes contaminated with o-bromostyrene produced with loss of HBr from the 1-bromo by-product. The problem of HBr formation during distillation can be alleviated somewhat by adding an HBr scavenger such as an epoxy resin to the distillation pot. However, such scavengers are not completely effective (a nonvolatile epoxy resin will not scavenge HBr formed in the column rather than in the pot) and, in any case, they do not solve the problem of o-bromostyrene contamination of the distillate.
It is therefore an object of the invention to provide a method for the recovery of a cyclobutenohaloarene from a reaction product mixture containing by-product halides. In a specific aspect, it is an object of the invention to provide a method for recovering 4-bromobenzocyclobutene from a reaction product mixture containing by-product aliphatic halides without generating large quantities of additional HBr and o-bromostyrene.